1. Field of the Invention
This invention relates generally to apparatus used in the manufacture of integrated circuits and more particularly to devices used to apply a pellicle to an integrated circuit photomask.
2. Description of the Prior Art
To produce integrated circuits, thin, polished, disk shaped wafers of semiconducting material (such as silicon) are coated with a photoresist material. The wafers are then exposed to light through an integrated circuit photomask, developed, and further processed.
Typically, a series of photomasks are used to produce the various layers of the integrated circuit. It is necessary that each photomask be properly positioned with the wafer so that the successive layers are properly aligned.
As integrated circuits proceeded from small scale integration (SSI), to medium scale integration (MSI), large scale integration (LSI), and very large scale integration (VLSI) the need for proper alignment of the photomasks with the wafers became ever greater. Also, as the density of the circuitry of the integrated circuit increased, it became increasing critical to keep the photomasks absolutely clean. By way of example, a single dust mote on the surface of a photomask can ruin the integrated circuit being developed on the wafer below it.
To combat these problems, the industry developed pellicles to protect surfaces of the photomask. These pellicles include a thin, transparent membrane stretched across a frame. The frame serves to hold the membrane rigid, and spaces the membrane from the surface of the photomask. A sticky adhesive is used to bond the frame to a surface of the photomask. Sometimes the pellicle is only attached to a front surface of the photomask (which is usually the most critical surface), but quite often pellicles are attached to both sides of the photomask.
If the pellicles are properly attached to the photomasks, they can be useful in providing proper alignment between successive photomask exposures. Also, the pellicles keep dust, oils, etc. off of the surface of the photomask, which greatly reduces the chance for contaminating the photomask. Since the membrane of the pellicle is separated from the surface of the photomask by the thickness of the pellicle frame, any dust or other contamination of the surface of the membrane will be out-of-focus relative the surface of the photomask, and thus will not ruin the exposure.
Great difficulties have been encountered in properly attaching pellicles to a photomask. Traditionally, this job was done manually, or perhaps with the aid of simple hand tools and binocular microscopes. However, traditional techniques for attaching pellicles to photomasks have been more of an art than a science, and costly mistakes were inevitable.